Bicycle Quarterly, Spring 2013 issue, has an interesting selection of rolling resistance test data. I recommend buying this if you can find it. I subscribe to the magazine and read every article: I really enjoy it even though it's focused more on randonneuring than on racing bikes. You don't need to agree with everything written in a publication to enjoy the discussion if there's interesting data, as we know from Tour magazine's frame tests.

They ride a track with a Powertap, possibly with an SRM as well (in most recent tests), at a moderate speed (150 watts, resulting speed 23-28.5 kph) then use AnalyticCycling to extract a Crr, which they temperature-adjust using results from a control tire. This isn't the most scientific method possible, as they don't say how they adjust for Crr.. personally I like the Chung method which is similar but slightly different. They inflated tires to the Berto recommended pressure, which targets 15% drop (ie loss of rolling radius due to load = 15% of the tire radius measured laterally, on the rim). But the results are sufficiently comprehensive that they are of interest. An in any case, if I want to criticize their methods, I should do my own freakin' test.

The following numbers I pulled from a bar graph, without grid lines, by eye, so there's some potential for small error probably less than the testing precision.

The best tires are various out-of-production racing tubulars which are no longer available starting with a Challenge 33-622 tubular @ 0.25%. The Vittoria Open Corsa Evo CX 25-622 clincher (with butyl tubes) is 0.32%. The same tubulars were next @ 0.34%. Vittoria Rubino's 25-622 were next @ 0.40%. Then Michelin Pro2 Race 25-622 and Grand Bois 32-622 were @ 0.43%, Grand Bois 29-622 @ 0.44%, Grand Bois 25-622 and Challenge Paris-Roubaix 27-622 @ 0.45%, Michelin Pro2 Race 23-622 and Continental Ultra Gator Skin 23-622 at 0.47%. I'll leave it there, but the list continues to an airless tire 28-622 which measured at an incredible 1.46%.

Other lessons:1. Air pressure has remarkably little effect on smooth roads, but on rough roads helps. Tubulars can be run at lower pressure, so this is an advantage, even if they don't necessarily have lower rolling resistance at the same pressure based on the results from the Vittoria.2. Narrow tires don't lower rolling resistance.

The difference between these results and roller tests is credited to suspension losses, since at higher pressures the rider bounces more, which is lossy.

I was shocked at how poorly the Michelin Pro Race 2 tires did. I have Pro Race 4 tires and really like them. That the Pro2 would test close to Gator Skins, which are generally acknowledged to be slow, is surprising.

I hadn't realized the Corsa CX's were available in 25C. Those, it seems, are the winner here. But more generally that fat tires, even 32mm, can be quite fast.

Do they test multiple tires of the same model, and even from different production lots? My guess is no. How much variation is there from tire to tire and lot to lot of the same model? Might this be more significant than some of the supposed differences between models? And then of course there is the environmental history (aging, temperature and humidity of storage, etc.) of the tires (and tubes) being tested.

They test different sizes in the same model, which provides some degree of control, since the different models seem fairly well correlated. I wonder if there's a significant difference between the Michelin Pro2 and later models. Notable is the Grand Bois tires don't do especially well, since the author of the study, Jan Heine, sells Grand Bois tires through Compass Cycles, speaks to his credibility. I actually really like Grand Bois tire: they mount well, handle well, and have good durability. I was disappointed to see them test so relatively poorly. The "Extra-Leger" model is of greater weight-weenie interest, and wasn't tested here.

One thing is with tubulars Al Morrison has documented that gluing technique has a strong effect, and so I would have liked to see this documented. It's hard to draw conclusions about tubulars versus clinchers otherwise.

They ride a track with a Powertap, possibly with an SRM as well (in most recent tests), at a moderate speed (150 watts, resulting speed 23-28.5 kph) then use AnalyticCycling to extract a Crr, which they temperature-adjust using results from a control tire. This isn't the most scientific method possible, as they don't say how they adjust for Crr.. personally I like the Chung method which is similar but slightly different. They inflated tires to the Berto recommended pressure, which targets 15% drop (ie loss of rolling radius due to load = 15% of the tire radius measured laterally, on the rim). But the results are sufficiently comprehensive that they are of interest. An in any case, if I want to criticize their methods, I should do my own freakin' test.

I'm glad they've switched to this kind of test, though of course I'm guessing that if the protocol is exactly as you describe their precision isn't very good. Here's how Heine described his Crr testing protocol last year, which involved a hill coast down, two spotters spaced 184m apart, and an unspecified way to synchronize the timing devices.

My protocol is pretty different because it's not done at either constant speed or constant power.

Some readers might find a Crr of 0.34% more familiar as .0034, but it can be handy if you recognize that Crr scales exactly like a slope, so an additional 0.1% in Crr is just like a slope which is 0.1% steeper.

Crr is the drag force divided by total force perpendicular to the ground, or more often just as dividid by total weight (on flat ground they are the same).

The Pro2 tests might be old. BQ has been doing tests for years, and these were a compendium. Still, I find those numbers surprising. I don't know the difference between Pro2 tires and Pro4.

In contrast, Contintental Gatorskins?

They have also done roll-down tests on a soapbox derby race track. Here they relied on gravity to provide the power. There would be a small error from differences in rolling mass as a % of total mass... but that's small.

They claim to have used a set of reference tires as a control but comparing tests done at different times is tricky. Differences in wind resistance are also a factor, as was noted, but these tests were all at relatively low speed where tire rolling resistance is still important. Variability in human rolling resistance is a greater concern, perhaps. So I think it's good to look for trends in tests like these (for example, fatter tires aren't slower; relatively lower pressures aren't as slow as one might think) rather than focus too heavily on the one particular number. But the CX Evos have consistently tested well in tests, these included, so I think it says a lot for those tires, which I hadn't realized were available in 25's.

I have a very old Pro2 Race that I use as a control tire, and it tests quite fast--faster than a one year old Conti 4000S. OTOH, I have an Axial Pro 700x25 (the predecessor to the Pro2) and it's actually worse than a Gatorskin. It's immediately apparent when riding it that it's really, really slow.

So, if you have any questions about a specific tire, your best bet is just to test the exact tire. In terms of variability among tires, my experience is that all of the Lion Tyre Company tires (Vittoria/Bontrager/Zipp/Ritchey etc) are remarkably consistent (although there are significant differences among the various tires they make, obviously).

As far the gluing methods making a difference with tubulars, I recently bought a used Stinger 6 with a VittoriaEvo CX 23, glued about 1 year ago with Mastic One. I'd describe it as "poorly glued", with some of the edges easily visible if I pulled up the edge of an uninflated tire. Not unsafe, but not fast. Crr was .0039 on my rollers. After gluing it properly to minimize Crr, it dropped to .0031. So yeah, gluing method makes a huge difference. A well-glued mediocre tire can be faster than a 'fast', poorly glued tire in my experience.

Curious: I was browsing Google on the subject and came across this result, using a pendulum whose damping rate measures rolling resistance. There's a comparison between Continental GP 3000's (generally considered good tires) and Michelin Axial Pros:

I don't know about the GP3000s, most pre-black chili Continentals were not fast rollers. The GP 3000 in 20c with butyl tube on AFMs charts is not good, in fact it's DFL(but most clinchers weren't tested with a butyl tube). But to the point of your chart, if it had a latex tube it would have been just slightly worse than the 23c Axial in his tests. And that margin may have been smaller if it had been a 23.

Thanks DJ, lots of good info.Doesn't the Vittoria Evo CX have a really bad reputation for wet grip? I seem to recall having read here on weightweenies that pros refused to use it, leading to the creation of the SC. And indeed all the Vittoria neutral service cars at last year's Tour of Austria had wheels with SCs on (the only race I went to see a stage).

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